1. Field of the invention
The present invention is generally concerned with mechanical filter screens designed to separate out from flowing water debris or waste materials carried by the latter.
These may be mobile filters, for example chain filters by which is meant filters in which the filter cloth moves in a closed loop or drum filters by which is meant filters in which the filter member is a rotating drum, or static filters such as those designed to be inserted into a pipeline, for example.
2. Description of the prior art
A common feature of such screens or filters is the use of one or more filter panels that the water to be treated has to pass through, the filter panel or panels comprising to this end appropriately sized openings.
At present such filter panels are generally two-dimensional, whether they are of mesh form, for example metal mesh with braided or welded wires, or of perforated sheet metal.
In other words, in the direction perpendicular to the plane of their openings they have no significant dimension other than that necessary to confer on them the appropriate mechanical strength.
Holding back debris or waste materials carried by the water flowing through them, they are progressively clogged by the latter, to the detriment of their overall effectiveness.
It is therefore necessary to clean them systematically or periodically.
In practise, such cleaning is done by passing water through them in the opposite direction: in the case of washing in air water jets are specifically employed for this purpose, whereas in the case of cleaning under water the direction of flow of the water is reversed by means of a suction arrangement.
The debris or waste materials previously held back by a filter panel are more easily removed by such counter-current circulation of water if they are not strongly adherent to the filter panel.
Strong adherence is often found in practise, however, and may be explained by either of the following processes or a combination of them both.
In the first process, when a long, thin and relatively rigid item such as a pine needle, for example, enters an opening or mesh in the filter panel through which it has to pass, it may be inclined relative to the plane of the opening and lie across the opening, so blocking it, in particular where the opening has a contour with corners as is the case with woven or welded metal mesh; there is in fact a natural predisposition for such jamming to occur, or even a wedging effect between two wires where they cross over.
In this case the counter-current circulation of water for cleaning serves only to press such debris or waste materials against the filter element, as they have already passed part-way through the filter panel and are locally jammed or wedged into it. The end result is that they cannot be detached from it by this means.
Thus such debris or waste materials are not removed, which is the opposite of the required effect.
In the second adhesion process that may be operative, flexible fibers of greater or lesser length carried in the flow of water to be treated "straddle" the solid members (wires, bars or the like) delimiting the openings in a filter panel; these may be textile fibers or other items similar to such fibers, for example certain filament-shaped algae or plant leaves in a relatively advanced state of decomposition.
Because of the turbulence that is inevitable on passing through a filter panel and because of the counter-current flow of water that occurs on reversing the flow for cleaning, a fiber of this kind may become wrapped around a solid member of this kind or may become tied into a knot on the far side of it; this possibility is facilitated in that, as already mentioned, the solid member concerned has no significant thickness with the result that fibers can easily have a length twice this thickness and therefore quite sufficient for the wrapping or knotting process in question to occur.
In either case, the counter-current circulation during cleaning may not suffice to detach from the filter panel the flexible fibers that it has held back, which leads by a progressive process often referred to as "felting" to more or less complete obstruction of the filter panel.
A third possible adhesion process concerns viscous bodies such as jellyfish or debris from such aquatic animals.
Held back by a filter panel, any such viscous body inevitably blocks one or more openings therein.
Because of the thrust on it due to the head loss that occurs on passing through the filter panel, it is deformed and at least part of it enters into the relevant opening in the filter panel; once it has passed through, it tends to resume its initial configuration, with average transverse dimensions greater than the average width of the opening.
It therefore forms, beyond the filter panel, a swelling resembling in some ways a rivet head.
As previously, the counter-current flow during cleaning is unable to push back this swelling and therefore cannot detach the viscous body concerned from the filter panel.
A general object of the present invention is an arrangement adapted to minimize or even eliminate totally the possibility of adhesion resulting from such processes.